Inter-ocean dispersal is an important mechanism in the zoogeography of hakes (Pisces: Merluccius spp.)

Aim To present new genetic data and to review available published genetic data that bear on the phylogeny of hakes in the genus Merluccius. To construct a zoogeographical model from a summary phylogenetic tree with dated nodes. To search for an explanation of antitropical distributions in hakes. To assess peripheral isolate, centrifugal and vicariance models of speciation in view of the molecular phylogeny and zoogeography of hakes. Locations Northern and southern Atlantic Ocean, eastern Pacific Ocean, South Pacific Ocean. Methods Electrophoretic analysis of 20 allozyme loci in 10 species of hakes. Phylogenetic tree construction with parsimony and bootstrap methods. Reanalysis of previous genetic data. Analysis of zoogeographical patterns with geographical distributions of molecular genetic markers. Results Phylogenetic analyses of new and previous allozyme data and previous mitochondrial DNA data indicate a deep genetic partition between Old- and New-World hakes with genetic distances corresponding to 10-15 Myr of separation. This time marks a widening rift between Europe and North America and a rapid drop in ocean temperatures that subdivided an ancestral population of North Atlantic hake. Two Old-World clades spanning the equator include pairs of sister taxa separated by tropical waters. Divergence times between these pairs of sister-taxa variously date to the early Pliocene and late Pleistocene. Amongst New-World hakes, pairs of sister taxa. are separated by equatorial waters, by the Southern Ocean, and by the Panama Isthmus. These genetic separations reflect isolation by the rise of the Isthmus 3-4 Ma and by Pliocene and Pleistocene dispersals. Pairs of species occurring in sympatry or parapatry in six regions do not reflect sister-species relationships, but appear to reflect allopatric divergence and back dispersals of descendent species. Some geographically isolated regional populations originating within the last few hundreds of thousands of years merit subspecies designations. Conclusions Vicariance from tectonic movement of continental plates or ridge formation cannot account for the disjunct distributions of most hake sister taxa. Molecular genetic divergences place the origin of most hake species diversity in the last 2-3 Myr, a period of negligible tectonic activity. Distributions of many hake species appear to have resulted from dispersals and back dispersals across both warm equatorial waters and cool waters in the Southern Ocean, driven by oscillations in climate and ocean temperatures. Genetic and ecological divergence prevents hybridization and competitive exclusion between sympatric species pairs in six regions. Sister-taxa relationships and estimates of divergence are consistent with the modified peripheral isolate model of speciation in which vicariances, range expansions and contractions, dispersals and founder events lead to isolated populations that subsequently diverge to form new species.